advice:
bind a useful key (like E or Q) to use medikits/bandages.
do not carry too much weight; it makes you slower AND use more stamina when running.
grab a decent gun asap; the 5.56 models are better than the 7.62. save your AP ammo.
sell (almost) all the artifacts - they are your best bet to get enough $$$ to buy a good armor. you will know which ones to keep.
once you have a decent armor (Bulat) and a scoped, suppressed TRs 313, you will be good for 90% of the game.
it does wear out quickly and starts to jam easily when dirty, but you should not need to use it that much - headshots and stealth will take care of most stuff.
you really want to engage long distance as often as possible; close combat will leave you bleeding in a second, and you are no more effective anyway.
Tap-firing is the way to go; do not fire full auto. You have a key that selects your fire mode. Also, full auto ruins guns faster.
The AK family of guns is meh; the short one won't hit anything past 5 feet and the full sized one is inferior to the TRs on range, accuracy, cost of ammo, and unreliable to headshot.
If you find a IL86, use it until you get a TRs, but don't spend money on it.
The Thunder is actually a good gun, hard hitter, but has a small magazine, and you only get it when you already have the GP37 or the equally viable SGi-5k, which are your end-game guns. Also uses weird ammo, and you get caught reloading EVERY F**** TIME.
The FT-200M is unmoddable and has more recoil than the SG, TR or GP. The SG is my preferred gun, it is almost identical to the GP, can be modded for 100% accuracy, and has a better scope. The GP has a weird scope, hits slightly harder but has slightly inperfect aim at long distance.
ALWAYS mod guns for less recoil, higher accuracy, and reliability. Never mod guns for rate of fire or damage, you will get a useless weapon that can't hit anything and jams immediately.
The SVD family is .. useless. One of the two is not 100% accurate, the other one is, but it's also heavy, expensive, and totally redundant because a fully kitted out TRs has less recoil, same pinpoint accuracy, same level of deadliness, you can use it at close range, and it uses normal ammo.
The Vintars are hard hitters, but they have the worst projectile trajectory ever. Use them only for stealthy CQC or better yet, sell them.
Once you get your exoskeleton, you won't have to worry about nothing. Just explore and kill everything you find.
When starting out, just avoid firefights; my best run in CS i just found a bunch of Military guys fighting some stalkers, and i grabbed an AK from the ground that one of the guys had. Just scout, find artifacts, loot everything, sell and buy a TRs.
You CAN run from firefights; if you are doing a "help us" mission, just hang back and pewpew with your pistol to provide some moral support (because your guns are useless anyway). You will get your bonus and not lose money on bandages / medikits / ammo / repairs. Seriously though, shotguns, don't even look at them.
They might be handy with a few mutants but are useless anywhere else. You should really not consider attacking anything until you have at least a Viper (9mm SMG), and even that is pathetic and can only kill from close range.
Don't be afraid to make multiple trips to pick up and sell enemy guns.
As soon as you find a TRs / AK, sell your pistol and its ammo - you need to save on weight.
I have never bothered with explosives, they just never hit anything, cost too much money and should be sold, and they weight. The RPG (yes, it's in the game) and the grenade launcher are useless and weight a shitton.
Keep your Army and Scientific medikits.
Get a Tier-3 detector ASAP. Don't be ashamed of quicksaving a lot.
That's all, you should be good to go now.